Voltage cutoff circuit for imploded picture tube

ABSTRACT

A resistor is connected in series with the heater coil of a kinescope tube. Hence, when the tube implodes and the temperature change causes the heater resistance to decrease, the current in the resistor increases. The base-emitter junction of a transistor is connected across the resistor such that the transistor conducts when the current in the resistor increases due implosion. Conduction of the transistor energizes a relay control in its collector circuit, which in turn opens a normally closed switch in the power supply circuitry. Thus, implosion causes the ultor power to be turned off.

XR 3e8009083 United States Patent Wright Mar. 26, 1974 VOLTAGE CUTOFF CIRCUIT FOR IMPLODED PICTURE TUBE [75] inventor: Arden Bernard Wright, Ocean 7 Twp.,.Monmouth County, NJ. [73} Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Aug. 22, 1972 21 App]. No.: 282,871

[52] US. Cl. l78/7.S R, l78/DIG. ll [5 1] Int. Cl. H04n 5/44 [58] Field of Search 178/75 R, DIG. ll;

315/117-128, DIG. 7, 94-107; 328/8-11; 323/42 [56] References Cited UNITED STATES PATENTS 5/1940 Linsell l78/7.85

3,419,807 l2/l968 Hursh et al, 328/ Primary Examiner-Richard Murray Attorney, Agent, or Firm-D. L. Hurewitz 57] ABSTRACT A resistor is connected in series with the heater coil of a kinescope tube. Hence, when the tube implodcs and the temperature change causes the heater resistance to decrease, the current in the resistor increases. The base-emitter junction f a transistor is connected across the resistor such that the transistor conducts when the current in the resistor increases due implosion. Conduction of the transistor energizes a relay control in its collector circuit, which in turn opens a normally closed switch in the power supply circuitry.

Thus, implosion causes the ultor power to be turned off.

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HIGH VOLTAGE CIRCUIT POWER SUPPLY VOLTAGE CUTOFF CIRCUIT FOR IMPLODED PICTURE TUBE BACKGROUND OF THE INVENTION This invention relates to video receiver circuitry. More particularly, it relates to apparatus for protecting against electrical shock which might result from imploded kinescope tubes.

Kinescope tubes, more commonly known as television picture tubes, are constructed as evacuated glass enclosures. The inner surface of the tube is coated with phosphors which in turn may be coated with a thin sheet of aluminum, and outside the backportion of the tube is a coating of a graphite-type material commonly known as aquadag. In the tear or neck of the tube is a heater which, when powered electrically, energizes electron guns. Whenever a sufficiently large voltage, known as the ultor voltage, is maintained between the inner coating of the tube and the outer aquadag coating, electrons from the guns are attracted toward the phosphors. There electrons impinging upon the phosphors from theguns cause the screen to be illuminated, thereby producing the video picture. In kinescope tubes used in the PICTUREPHONE video telephone service, the ultor voltage between the aluminized-phosphors and the aquadag is approximately 12,000 volts; in commercially available color television sets, the ultor voltage may be twice as large.

As long as the kinescope tube remains intact, the high ultor voltage presents no safety hazards. Any intrusion into the inner portion of the tube occasioned by or after an implosion of the tube. however. could result in serious electrical shock hazard from the ultor voltage. For example, it is not at all difficult to imagine a young child watching the set and becoming either curious as to how the picture is produced or perhaps enraged at the downfall of one of its television heroes. In such a case, the child might be moved to strike the picture tube with a toy of sufficient bulk to fracture the screen and thereby cause the kinescope tube to implode. Having done so, the child very likely may reach in, either to retrieve the toy or merely to examine the internal space created by the implosion. In either case, there is strong likelihood that the child would be exposed to many thousands of volts, certainly a potentially dangerous situation.

The prior art shows several schemes which are designed to reduce the possibility that tube implosion will occur. For example, if belts of metal held under tension are banded around the periphery ofa kinescope, stress which is applied to the screen will be largely absorbed by the belts and the possibility of implosion will be reduced. Another scheme has attempted actually to strengthen the screen of the kinescope tube. In some models of the PICTUREPHONE service station set, a glass shield is epoxied onto the face of the tube, thereby giving it increased structural strength. "niie both schemes are helpful insofar as they reduce the possibility of implosion occurring, neither is entirely effective and neither addresses itself to the danger of electrical shock which results when implosion does occur.

SUMMARY OF THE INVENTION The present invention obviates the possibility of electrical shock after a kinescope implosion by turning off the utlorwvoltage- More particularly, the present invention is based on the proposition that when implosion occurs, air rushes into the tube and the interior of the tube is no longer evacuated. This influx of air causes a reduction in the temperature of the tube heating coil, a reduction which in turn decreases the electrical resistance of the heater coil. Since the voltage on the heater coil is unchanged by the implosion, the result of implosion is therefore an increase of current flow in the heater coil circuit. The present invention provides that when the current in the heater coil circuit increases above a point which indicates that implosion has occured, switching circuitry is activated and the ultor voltage is thereby turned off.

In an illustrative embodiment of the present invention, a resistor is connected in series with the heater coil of a kinescope tube such that any increase in coil current causes an increase in potential drop across the resistor. More particularly, the series resistor is connected between the base and emitter of a transistor, and the magnitude of the resistor is selected such that the transistor turns on whenever coil current increases beyond a selected point, the current corresponding to tube implosion being in excess of the selected point. Thereupon, a relay control in the collector circuit of the transistor is energized, and a relay switch is opened such that the ultor voltage is turned off.

It is a feature of the present invention that shock danger from imploded picture tubes are obviated by means ofa temperature sensitive process. Moreover, this temperature sensitivity is apprehended by an electronic circuit which is extremely simple and inexpensive in terms of embodiment. Thus, nearly all kinescope tube circuits could easily incorporate the principles of the present invention and thereby substantially enhance their operational safety with scarcely any increase in manufacturing costs.

SUMMARY OF THE DRAWINGS FIG. 1 shows a kinescope tube which has no safety protection of the sort incorporated by the principles of the present invention;

FIG. 2 shows a heater current versus voltage characteristic of evacuated and imploded kinescope tubes;

FIG. 3 shows a kinescope tube with one embodiment of the present invention included therein; and

FIG. 4 shows a kinescope tube with another embodiment of the present invention included therein.

DETAILED DESCRIPTION FIG. 1 shows a symbolic diagram of a standard video kinescope arrangement without any protection against implosion shock hazards. The kinescope itself constitutes an evacuated glass case 101 of familiar picture tube shape. On the outer surface of the tube, excluding the screen portion, is the aquadag coating 102. Likewise, the aluminized phosphors 103 line the interior of the tube. A voltage is applied in the heater circuit 104 at the neck of the tube, thereby energizing the electron guns. The power supply for the set is represented merely by a functional block 108 which feeds, among other things, the high voltage circuit 107. The high voltage circuit 107 produces the ultor voltage across lines 105 and H16. Hence, whenever the ultor voltage is produced across Wires 105 and 106, the phosphors 103 are energized to be illuminated by impinging electrons from the electron guns.

It is apparent that the apparatus of FIG. 1, in the case of an impact on the screen portion of the tube of sufficient force to cause implosion, provides nothing in the way of shock prevention. That is, even though the front of the tube is penetrated, the very high ultor voltage between the aquadag 102 and the phosphors I03 continues in an on condition. Normally, the aquadag coating 102 is at ground and the phosphors 103 are at a positive voltage. Thus, anyone touching the inner surface of the phosphors provides a path to ground for the 12,000 or more volts of the ultor voltage. It is just such'a hazard which the principles of the present invention are designed to obviate.

The present invention is based on the discovery that the heater circuit of a kinescope tube reacts in certain predictable ways after implosion. FIG. 2 shows a graph for tubes of the sort used in PICTUREPHONE service sets of DC heater voltage plotted against heater current for conditions of a normal evacuated tube and for an imploded tube. Heater current is represented on the ordinate, measured in milliamperes, and heater voltage is represented on the abscissa, measured in DC volts. The dotted line 201 projects heater current against voltage in an evacuated kinescope tube. In the normal case, therefore, the heater circuit responds along the dotted line characteristic. In the case of implosion, the environment within the tube is changed radically, being drawn from a vacuum to air essentially at atmospheric temperature. In such a case, the heater voltage versus current characteristic changes to the solid line 202 plotted in FIG. 2. Clearly, substantially more current flows when the tube is imploded as compared to its characteristics in an evacuated state, ie, the coil resistance decreases. In the caseof an AC power supply for the heater, the curves 201 and 202 would maintain the same relative shape and only the current and voltage magnitudes would change. It is this phenomenon upon which the principles ofthe present invention are based.

FIG. 3 shows a kinescope identical to the one shown in FIG. 1, with the exception that the FIG. 3 circuit includes an embodiment of the principles of the present invention as applied to a DC powered heater circuit. In FIG. 3, tube casing 301, aquadag 302, phosphors 303, heater coil filament 304, power supply 308 and high voltage circuit 307 operate identically to similarly numbered items of FIG. 1. In order to apprehend and to uti lize the changed current characteristic demonstrated by FIG. 2, a resistor 309 is connected in series with the heating coil filament 304. Connected across the series resistor 309 is the base-emitter junction of a transistor 310. In the collector circuit of the transistor 310 is a switch control 311 which regulates the state of a normally closed switch 312.

The embodiment of FIG. 3 operates as follows. The size of resistor 309 is chosen such that the current in the heater filament 304 and therefore also through the resistor 309 during normal conditions is insufficient to turn on transistor 310. On the other hand, it is desirable that when implosion occurs, the increase in current through the filament 304 causes sufficient increase in voltage across resistor 309 to turn on transistor 310. For example, for the kinescope tubes currently utilized in PICTUREPHGNE service station sets which are characterized by the current versus voltage characteristic of FIG. 2, 5 ohms is an appropriate magnitude for resistor 309. Then the heater circuit is operating with a voltage supply of 8 volts, approximately I00 milliarnperes flow through resistor 309, and the base-toemitter voltage of transistor 310 is 0.5 volt. This potential of 0.5 volt is insufficient to forward bias transistor 310 into conduction. Clearly, therefore, so long as the tube is evacuated, transistor 310 remains in an off condition. Whenever implosion occurs, however, the current through the 5 ohm resistor 309 increases to approximately l milliamperes, which increases the voltage across resistor 309 well above that necessary to turn on transistor 310. When this occurs, current flows in the collector circuit oftransistor 310, the switch control 311 is thereby energized, and the switch 312 is opened. This opening of switch 312 causes the ultor voltage to be removed from across the aquadag 302 and the phosphors 303, and the possibility of electric shock derived from anyone touching the phosphors 303 is obviated.

In summary, the embodiment of FIG. 3 operates by sensing in resistor309 the current in the heater filament 304, whereby an increase of sufficient magnitude of heater current turns on transistor 310. When this occurs, switch control 311 is energized and switch 312 is opened, thereby removing the ultor voltage from the tube.

The embodiment of FIG. 3 operates for heater circuits which utilize DC voltage supplies. By means of a simple adaptation, shown in FIG. 4, the protection circuitry embodying the present invention can readily be made to operate for heater circuits which utilize AC voltage supplies. The embodiment of FIG. 4 operate in a manner similar to the embodiment of FIG. 5 and like functioning elements are identified by numbers having the same last two digits in both figures. However, differences do exist since the voltage supply 415 is an AC. source. First, a diode 413 is connected in series between the heater coil filament 404 and the resistor 409, and the AC variations in heater current are thereby rectified. Then, a capacitor 414 is connected across resistor 409 (and therefore also across the baseto-emitterjunction of transistor 410), and any remaining fluctuations in the rectified voltage across resistor 409 are smoothed. In this fashion, notwithstanding any AC variation in the heater circuit, transistor 410 is subjected to uniform base-to-emitter voltages, either below or above the voltage required to turn it on. Of course, since the scale factors of the heater voltage versus current characteristic are changed in the AC case from that shown in FIG. 2, the magnitude of resistor 409 is different from that of resistor 309 in FIG. 3.

The switch 312 along with its control 311 is embodied in FIG. 3 as a relay type switch. It is apparent, however, that any number of switches well known in the art could be substituted therefore. Likewise, the use of a transistor 310 constitutes but one of a number of types of switches which could be activated under the conditions prescribed in accordance with the principles of the present invention. Finally, while the embodiment of FIG. 3 operatesresponsively to a condition of implosion to turn ofi only the ultor voltage, it is well within the scope of the present invention that the entire power to the set could be turned off as well. In fact, such a case might even be preferable under certain circumstances, depending upon the source from which the set power=is derived.

What is claimed is:

l. in a video receiver set, apparatus comprising, in

combination:

a kinescope tube including a heater filament characterized by an electrical resistance directly proportional to temperature, said filament being energized to produce heat within the kinescope tube;

means connected to said filament for detecting the presence of a predetermined amount of'current in said heater filament indicative of the presence of air at atmospheric temperature in said tube, occasioned by implosion; and

means responsive to said means for detecting for disconnecting power from said kinescope tube.

2. in a video display set, apparatus comprising, in

combination:

a kinescope tube having an electrically powered heating element characterized by an electrical resistance which is directly proportional to ambient temperature, said element being energized to produce heat within the kinescope tube, and having electrically powered high voltage electrodes;

said heater element being connected to current sensing means operative when the current in said heat ing element increases beyond a predetermined le el, said predetermined level being lower than the current which results from the cooling of said heating element due to implosion of said kinescope tube; and

switching means activated by said current sensing means for disconnecting operating power from said high voltage electrodes.

3. Apparatus as described in claim 2 wherein said current sensing means and said switching means include a transistor and a resistor connected between base and emitter of said transistor so that a sufficient bias voltage is established across saidv base and emitter to switch said transistor into a state of conduction upon an increase of current in said heating element to a level greater than said predetermined level.

4. Apparatus as described in claim 3 and further comprising a diode connected in series between said resistor and said heating element and a capacitor connected in parallel with said resistor, said diode and said capacitor rendering said current sensing means responsive to alternating current in said heating element. 

1. In a video receiver set, apparatus comprising, in combination: a kinescope tube including a heater filament characterized by an electrical resistance directly proportional to temperature, said filament being energized to produce heat within the kinescope tube; means connected to said filament for detecting the presence of a predetermined amount of current in said heater filament indicative of the presence of air at atmospheric temperature in said tube, occasioned by implosion; and means responsive to said means for detecting for disconnecting power from said kinescope tube.
 2. In a video display set, apparatus comprising, in combination: a kinescope tube having an electrically powered heating element characterized by an electrical resistance which is directly proportional to ambient temperature, said element being energized to produce heat within the kinescope tube, and having electrically powered high voltage electrodes; said heater element being connected to current sensing means operative when the current in said heating element increases beyond a predetermined level, said predetermined level being lower than the current which results from the cooling of said heating element due to implosion of said kinescope tube; and switching means activated by said current sensing means for disconnecting operating power from said high voltage electrodes.
 3. Apparatus as described in claim 2 wherein said current sensing means and said switching means include a transistor and a resistor connected between base and emitter of said transistor so that a sufficient bias voltage is established across said base and emitter to switch said transistor into a state of conduction upon an increase of current in said heating element to a level greater than said predetermined level.
 4. Apparatus as described in claim 3 and further comprising a diode connected in series between said resistor and said heating element and a capacitor connected in parallel with said resistor, said diode and said capacitor rendering said current sensing means responsive to alternating current in said heating element. 